Coasting arc ion source



Sept. 10, 1957 J. s. FOSTER, JR 2,

COASTING ARC fON SOURCE Filed July s, 1952 2/ nnrm nrmn 3/\ POWER II SUPPLY U U U U U U U U POWER 34\ SUPPLY I 33 'lh w l'l l T 2 INVENTOR. v JOHN s; FOSTER, Jr.

ATTORNEY.

United States Patent CDASTING ARC ION SOURCE John S. Foster, Jr., Albany, Calif., assiguor to the United States of America as represented by the United States Atomic Energy Commission Application July 8, 1952, Serial No. 297,629

19 Claims. (Cl. 313-63) The present invention relates to an improvement in ion sources and in particular to the efiicient removal or ejection of ions from an ion source.

Ion sources have been developed that ionize a large percentage of gas fed thereto with one of the most efficient types being the oscillating electron discharge type, as shown by Penning, Patent No. 2,197,079 and Smith, Patent No. 2,507,652. Efficient ion generation, however, must be accompanied by efiicient and practicable ion ejection to make any ion source usable and attempts to remove an appreciable quantity of theions generated in a source has met with considerable difiiculty. In this respect, there have been developed a variety of ion ejection means, as for example the devices of Backus, Patent No. 2,499,289 and Loevinger, Patent No. 2,499,320; however, there remains a major limitation of a rather fundamental nature which operates to preclude large scale ion ejection by conventional means. This limiting factor may be expressed as a ratio of the amount of unionized gas to ionized gas that leaves the ion source. Substantially all apparatus employing ion beams operate upon the beams in a vacuum, and thus the benefit or advantage of an increased amount of ions from a particular ion source is directly offset by an increase in gas flow from the source into whatever evacuated chamber may be provided for accepting the ions produced in the source. A further point of consideration is the relative unavailability of the ions disposed near the center of the arc plasma produced by an oscillating electron ion source for example, for conventional ion ejection means adapted for use with this type of source operate to remove the ions forming the exterior portion of the arc plasma and fail to remove ions from the entire arc cross section.

The present invention provides an improved ion source adapted to overcome the above-noted limitations and difficulties commonly associated with are sources and to eject ions from the arc plasma in proportion to the amount of ions comprising same by the provision of a coasting arc, as explained below. The capabilities are attained by the accomplishment of the following principal objects:

It is an object of the present invention to provide an improved oscillating electron ion source.

It is another object of the present invention to provide an improved method and means of removing ions from an ion source.

It is a further object of the present invention to provide an improved ion source having a coasting arc of expanded cross section from which ions are removed.

It is still another object of the present invention to provide an improved oscillating electron ion source, wherein ions are removed from the end of the arc plasma at an expanded cross section thereof.

Many further objects and advantages will become apparent from the following description and claims taken together with the attached drawing wherein:

Figure 1 is a cross sectional view of one preferred embodiment of the invention; and

Figure 2 is a schematic wiring diagram of the embodiment of the invention shown in Fig. 1.

ice

Considering now the elements of the illustrated embodiment of the invention and referring to Fig. 1, it will be seen that there is provided a cylindrical anode 11 formed of electrically conducting material and having open ends 12 and 13. Attached about anode cylinder 11 at one end 13 thereof is a member or flange 14 which may be mounted upon cylinder 11 as by set screws 16. Flange 14 is formed of a material having a good permeability or specific conductivity for magnetic fiux and is shaped to fit about the end of cylinder 11 and to extend backward from the end of cylinder 11 so as to have somewhat the configuration of a hollow truncated cone. Also disposed at end 13 of anode cylinder 11 is an exit grid structure 17 which is fitted into open end 13 of cylinder 11 in closing relation thereto. Exit grid 17 preferably has a substantial thickness, for reasons noted below, and is provided with one or more apertures 18 therethrough aligned parallel to the axis of anode cylinder 11. Exit grid 17 is formed of an electrically conducting material and is maintained in position by mounting upon cylinder 11 in any suitable manner, the details of which are excluded in the interest of simplicity.

Disposed at the opposite end 12 of cylinder 11 from exit grid 17 is an electron emissive cathode 19 which may assume either the filamentary configuration illustrated, or alternatively, any one of a variety of known configurations such as, for example, a cold cathode aluminum plate. Cathode 19 is disposed adjacent the open end 12 of cylinder 11 and in alignment therewith and in the illustrated embodiment is connected by suitable electrical leads across a pair of terminals 21 whereby energization of the cathode may be accomplished. A magnet winding 22 in the form of a long solenoid is disposed about anode cylinder 11 concentrically therewith and may be maintained in this position by suitable mounting (not shown) which may either extend from magnet windings 22 to anode cylinder 11 or to an enclosing source housing. When energized, magnet winding 22 produces a magnetic field having flux lines passing axially through cylinder 11 and identified in Fig. 1 by arrows labeled H.

A further portion of the improved ion source of the invention is the grid structure shown in part at 23 of Fig. 1 and disposed at a distance from the end 13 of cylinder 11. In the illustrated embodiment of the invention grid structure 23 includes first grid 24 and second grid 26 disposed symmetrically about an extension of the axis of anode cylinder 11 and being separated from each other. First and second grids 24 and 26 each have the same curvature to present their convex faces toward cylinder 11 and are disposed with first grid 24 directly intermediate anode cylinder 11 and second grid 26. First grid 24 is formed with out large aperture 27 through the center thereof and second grid 26 is formed with a plurality of smaller apertures 28 therethrough generally within the projected area of the aperture 27 of grid 24 upon second grid 26.

It is to be realized that the illustrated and described constitution of grid structure 23 is not mandatory and the limiting factors thereof are noted below in connection with the explanation of operation of the invention.

In addition to the above elements of the invention there are also provided a gas or charging system and a vacuum system. The gas system, not shown, is adapted to provide quantities of gas to the source for ionization and is connected to one end of a pipe or tube 29 which extends into anode cylinder 11 to conduct gas therein. The vacuum system, not shown, is preferably connected to the source intermediate anode cylinder 11 and grid structure 23 and is equipped with pumping apparatus to remove unionized gas escaping from the end of anode cylinder 11.

The above-described elements of the invention must be energized to properly perform their intended functions and, in this respect, attention is invited to Fig. 2 which illustrates a simple wiring diagram for the embodiment of Fig. 1. Cathode 19 may be energized by a current supply 31 connected across terminals 21. Also, cathode 19 is maintained at a negative potential Witlfrespect to anode cylinder -11 and for this purpose there'is provided a power supply, illustrated as battery 32, which is connected between anode 11 and cathode 19. First grid 24 is electrically connected to anode 11, which may be grounded, and a high voltage power supply, shown as battery 33, is electrically connected between anode 11 and second grid 26 with grid 26 being connected to the negative terminal thereof. Magnet windings 22, shown schematically in Fig. 2as a long solenoid,-areenergized by a magnet power supply 34 connected thereacross.

Considering now the operation of the invention and referencing same to the illustrated embodiment, it will be seen that with cathode 19 energized from current supply 31 and anode cylinder 11 maintained at a positive potential with respect thereto by voltage source 32 there will occur an electron discharge from cathode 19 toward anode 11. The energization of magnet windings 22 by magnet power supply 34 produces a magnetic field longitudinally through anode cylinder 11 which restrains the motion of the discharge from cathode19. The electrons emitted from cathode 19 are constrained to generally travel along the lines of force of the magnetic field and actually trace helixes longitudinally of anode cylinder 11 with the diameters thereof being dependent upon the relative anode-cathode potential and the magnetic field strength.

This discharge thus travels through anode cylinder 11 and on through the apertures 18 in exit grid 17. Exterior to anode cylinder 11 the magnetic field diverges sharply because of the low reluctance'path of flange 14 and the discharge is thus diffused by the diversing lines of force H of the magnetic field. As the discharge travels further from anode cylinder 11, it is thus expanded by the diverging magnetic field as it tends to follow the lines of force thereof, and it continues to diverge as it proceeds into a substantially magnetic field free region away from anode cylinder 11.

The discharge is thus materially expanded as it approaches grid structure 23 and in the vicinity of grid structure 23 the electrons thereof experience the repelling force of the electric field produced by the high negative potential of second grid 26 to such a degree as to reverse direction. a second repelling cathode or grid disposed at a distance from anode 11 so that the electron discharge is repelled therefrom and reverses direction to travel back toward anode 11 and in part to again traverse anode cylinder 11 until repelled by the negative potential of cathode'19.

It will be appreciated that there has been described above an oscillating electron discharge similar, in some respects, to that of a Phillips Ion Gage or the like; however, it is not vital to the present invention that the electron discharge oscillates, although this is perhaps the most eflicient method of ionization. This electron discharge has extremely good ionizing properties owing to the extended electron path and consequent high probability of ionizing collisions between discharged electrons and gas molecules. With the provision within anode cylinder 11 of a gas to be ionized, as for example through an inlet tube 29, the electron discharge will ionize a large quantity of ions to produce an arc plasma which will have substantially the same boundaries as the above-described electron discharge and which, for convenience, is indicated by the numeral 36 in Fig. 1. Ions are produced within anode cylinder 11 by electrons striking gas molecules therein and knocking off an electron from the molecules. The resultant positively charged ions in the arc plasma are also constrained by the magnetic field to gen- G-rid structure 23 in effect comprises by the combined area of the apertures 18 therein.

- 4 erally follow the lines of force thereof and thus the ions which are urged to the arc ends by the small potential gradient therein also travel through the apertures 18 in exit grid 17 and diverge. The are plasma therefore extends exterior to anode cylinder 11 and coasts in the magnetic field free region to the vicinity of grid structure 23. As the arc is coasting, it is also expanding as it follows the diverging lines of force of the magnetic field and the arc plasma therefore has a highly expanded area adjacent grid structure 23. The negative electric field of second grid 26 not only repels the electrons in the are but it also attracts the ions in the are so that the arc plasma terminates in this area with the negatively charged electrons reversing direction and the positively charged ions being urged out of the arc. The positive ions are forceably accelerated from the arc toward and through grids 24 and 26 by the highly negative accelerating potential placed on grid 26 by voltage source 33 and these ions are thus expelled for use in whatever type of apparatus may be attached to or operating in connection with the source, such as a particle accelerator or a mass spectrometer.

The above brief description generally covers the operation of the illustrated embodiment of the invention; however, various points are believed to merit particular consideration and thus attention is invited to the following. It has been found in connection with conventional sources of the Phillips Ion Gage type that the number of ions that may be removed from the end thereof varies with the diameter of a single opening therein, rather than the area, while the gas flow out the end is, of course, proportional to the area of the opening. This relationship apparently results from space charge effects and a diminution of the electric accelerating field at the center of an ion exit aperture as compared to the edges thereof. The result of such a relation is to preclude the use of larger ion exit apertures, inasmuch as the gas fiow therethrough increases more rapidly than the ion flow with larger apertures. Applicant has overcome this limitation by increasing the area of the end of the arc from which ions are expelled without increasing the area for gas flow. Thus, with reference to Fig. 1 it will be seen that the amount of gas flowing out of the end of anode cylinder 11 is limited by the cross sectional area thereof, or with exit grid 17 Although the are 36 is also limited by this area, ion expulsion is not made at this point but rather at an expanded arc diameter after the arc has coasted into the vicinity of grid structure 23. The unionized gas escaping from end 13 of anode cylinder 11 is removed by pumping means, not shown, between anode cylinder 11 and grid structure 23 while the arc plasma continues to travel away from anode 11. There is thus obtained a very advantageous ratio of area for unionized gas to escape to area from which ions may be removed.

A further consideration is the density distribution in the arc plasma cross section, for this materially affects ion removal. In general the arc plasma within anode cylinder 11 tends to have a greater density near the axis thereof and without correction this would result in a disadvantageous plasma density distribution at grid structure 23. If the center of the arc plasma were more dense at grid structure 23, then the accelerating electric field could not act uniformly upon the arc to remove ions therefrom. To obviate this difficulty exit grid 17 may be formed with more or larger apertures at a greater distance from the center, i. e., the ratio of open to closed area increases with radius. In this way the arc plasma emerging from anode cylinder 11 has a substantial uniform density distribution in cross section and the coasting arc presents a uniform end surface for ion ejection. In this respect it is noted that'second grid 26 is formed with a plurality of apertures, grid 26 preferably having "a large transparency, such as for example, and thus w ter ions are substantially equally attracted from the entire expanded end of the coasting are. In this manner the amount of ions accelerated or ejected from the arc is found to be substantially proportional to the area of the end of the are rather than the diameter as in more conventional sources. It will thus be seen that theoretically the ratio of ions removed to gas escaping is proportional to the ratio of the cross sectional areas of anode cylinder 11 and the expanded end of are 36. With an expansion of the coasting are to five times the diameter of the anode cylinder, it is thus theoretically possible to increase the ion ejection almost twenty-five times thatavailable, in the absence of a coasting are, without allowing any increase in gas flow. While this theoretical advantage has not been actually achieved in whole, there have been produced, without an increase in the volume of escaping gas, many times the number of ions that could be ejected without the coasting are.

It will therefore be appreciated that the coasting are provides a multitude of advantages which in'practical terms may be in part identified as follows: an expanded arc cross section permits the use of large electrodes which may therefore be easily cooled to minimize heating thereof; gas flow is limited to a smaller area than is ion ejection; better optical focusing may be produced by larger ion ejection grids; a greater ion current is possible in that an equivalent ion current from a small cross section would require larger accelerating voltages than it is possible to maintain in the accelerating gap therefor. In addition, many further advantages will be apparent to those skilled in the art.

In order to obtain optimum ion ejection it is necessary that a proper relationship be maintained between grid structure 23 and are plasma 36. This relationship re quires a uniform arc plasma density adjacent grid structure 23, as noted above, and is also dependent upon various other factors, such as proper arc expansion and uniform attraction of the ions in the arc plasma 36 by the electric field of the grid structure. These later factors are a function of the exit grid construction, the grid structure disposition, configuration, and composition. It has been found advantageous in this respect to provide curved grids which also produce a focusing action upon the emergent ion beam in that the electric field of second grid 26 is converging away from the source and therefore acts as a lens or the like to cause ejected ions to be directed to a focal point at some distance from the grid depending upon the radius of curvature thereof. Equal attraction of the ions from the end of arc plasma 36 is accomplished by the use of two or more grids, as shown. Thus, with a high negative potential upon second grid 26 and the anode potential upon first grid 24 there is produced a virtual cathode displaced from grid 24 toward anode 11 and of the same potential as cathode 19. This virtual cathode marks the end of arc plasma 36, for at this point electrons in the anc are repelled. Also by the use of one or more first grids 24 with one large aperture 27 therethrough the edges of the end of arc plasma 3% are sharply defined and the effective area of second grid 26 and the attractive force thereof is limited to the desired extent. it will be appreciated that a plurality of grids may be employed intermediate second grid 26 and anode 1i and in certain applications and with particular geometry, it may be found advantageous to employ more than the one grid 24 illustrated.

There has been described by the foregoing a preferred embodiment of the present invention incorporating the coasting or expanded arc provision thereof. From this description it will be seen that the present invention provides an improved ion source producing a large quantity of ions particularly with reference to the quantity of gas escaping therefrom. Inasmuch as the invention has been described only in connection with one embodiment thereof, it is not intended that it be limited to the details shown, for numerous embodiments are possible, and thus 3 attention is invited to the following claims for a defini tion of the scope of the invention.

What is claimed is:

1. An improved ion source comprising an electron emissive cathode, an apertured anode maintained at a positive potential with respect to said cathode to establish an electron discharge therefrom, means directing a gas to be ionized into said electron discharge to produce an arc plasma, a grid structure disposed on the opposite side of said anode from said cathode and maintained at a negative potential with respect to said anode for removing ions from said are plasma, and means expanding said are plasma adjacent said grid structure to provide a large arc plasma area for ion removal.

2. An improved ion source comprising means establishing an oscillatory electron discharge and including a cathode at one end thereof, means directing a flow of gas to be ionized into said discharge whereby said gas is ionized to produce an arc plasma, a grid structure disposed at the opposite end of said are from said cathode and being maintained at a negative potential with respect to said cathode, and means expanding said are plasma adjacent said grid structure to provide a large arc plasma cross section for ion removal.

3. An improved ion source comprising a cathode, an apertured anode maintained at a positive potential with respect to said cathode to produce an electron discharge, means directing a gas to be ionized into said electron discharge whereby said gas is ionized to produce an are, means establishing a magnetic field having lines of force directed between said cathode and anode to constrain said arc to pass through said anode, and a grid structure maintained at a negative potential with respect to said are disposed in alignment with said are on the opposite side of said anode from said cathode in a magnetically field free region whereby said are coasts to said grid structure and expands to an increased diameter to provide a large are cross section for ion removal.

4. An improved ion source comprising a cathode, an apertured anode disposed adjacent said cathode to produce an electron discharge therefrom, means directing gas to be ionized into said electron discharge to produce an arc, means establishing a magnetic field having lines of force extending from said cathode through said anode to constrain said are to extend through said anode, said magnetic field extending only a short distance beyond said anode, and ion removal means disposed on the opposite side of said anode from said cathode in a magnetically field free region in alignment with said are whereby said ion removal means are exposed to an expanded are cross section.

5. An improved ion source comprising a cathode and anode having a common axis, said anode having an aperture therethrough about the axis thereof, means directing gas to be ionized into a region intermediate said anode and cathode whereby an arc discharge occurs, means establishing a magnetic field between said anode and cathode parallel to the axis thereof whereby said are discharge is constrained to pass through said anode aperture, means disposed. adjacent said anode on the opposite side thereof from said cathode for diverting said magnetic field and causing same to diverge from the axis of said anode whereby said are is expanded exterior to said anode, and a grid structure maintained at a negative potential and disposed on the opposite side of said anode from said cathode about the common axis thereof and in the region of expanding arc whereby an expanded arc cross section is exposed to said grid structure for ion removal.

6. An improved ion source comprising a cathode, anode, and grid structure displaced along a common axis in the named order, said anode having an aperture therethrough about the common axis, means introducing a gas to be ionized intermediate said anode and cathode whereby an arc discharge is produced therebetween, means establishing a magnetic field along the said common axis and directing said are discharge through said anode, and means terminating the extent of said magnetic field along said common axis intermediate said anode and grid structure whereby said are discharge .expands on the opposite side of said anode from said cathode, and said grid structure being maintained at a negative potential to accelerate ions from the expanded end of said are discharge.

7. In an oscillatory electron ion source having a cathode, an apertured anode, a magnetic field extending therebetween through said anode, and means introducing a gas to be ionized between said anode and cathode whereby an arc discharge is produced to extend through said anode, the combination comprising means terminating the extent of said magnetic field on the opposite side of said anode from said cathode to produce a magnetically field free region wherein said are discharge radially expands, and a grid structure disposed in said field free region in alignment with said are discharge and having a negative potential impressed thereon whereby ions are attracted from said are discharge at an expanded cross section thereof.

8. An improved ion source comprising a cylindrical anode, a cathode disposed at one end of said anode, means establishing a magnetic field having lines of force extending axially through said anode, means directing a gas to be ionized into said anode whereby an are discharge is produced through said anode, means terminating the extent of said magnetic field external to said anode, and a grid structure having a negative potential impressed thereon disposed on the opposite side of said anode from said cathode outside of said magnetic field and aligned With said are discharge to attract ions therefrom.

9. An improved ion source comprising a cylindrical anode, means establishing a magnetic field axially through said anode, a cathode disposed at one end of said anodewhereby an electron discharge is produced and constrained by said magnetic field to travel through said anode, means directing a gas to be ionized into said electron discharge whereby said gas is ionized to change said electron discharge to an arc discharge, and a grid structure axially aligned with said anode and disposed on the opposite side thereof from said cathode external to said magnetic field, said grid structure being maintained at a negative potential to attract ions from said are.

10. An improved ion source comprising a cylindrical anode, means for producing a magnetic field axially of said anode, a cathode disposed at one end of said anode to produce an electron discharge that is constrained by said magnetic field to pass through said anode, means I directing a gas to be ionized into said electron discharge to change same into an arc discharge, a negatively charged grid structure axially aligned With said anode and displaced therefrom on the opposite side thereof from said cathode, and means for producing a magnetically field free region in the vicinity of said grid structure whereby said are discharge expands to present a large cross section for the removal of ions by said grid structure.

11. An improved ion source as defined in claim 10 further characterized by an exit grid disposed across the end of said anode away from said cathode and controlling the arc discharge extending therethrough toward said grid structure.

12. An improved ion source as defined in claim 10 further characterized by said grid structure comprising a plurality of parallel grids with the grid furthest from said anode having a plurality of small apertures therethrough and all other grids having one large aperture therethrough and being electrically connected to said anode.

13. An improved ion source comprising a cathode, an apertured anode axially aligned with said cathode, means for producing a magnetic field having lines of force extending from said cathode through said anode, means directing a gas to be ionized into the'region between said anode and cathode whereby said gas is ionized by electrons discharged from said cathode to produce an arc discharge extending through said anode aperture, means disposed on the opposite side of'said anode from said cathode about said anode aperture and having a high magnetic permeability to direct the lines of force of said magnetic field radially outward from an extension of the axis of said anode whereby said are discharge radially expands, a grid structure axially aligned with said anode on the opposite side thereof from said cathode and displaced from said anode to contact said are discharge in expanded cross section, and power supply means maintaining said grid structure at a negative potential to repel the electrons of said are and to attract ions from the expanded end thereof.

14. An improved ion source comprising a cathode, a grid axially aligned with said cathode at a distance there from and having a larger diameter than said cathode, a cylindrical anode axially aligned between said grid and cathode adjacent said cathode and displaced from said grid, power supply means maintaining said cathode at a negative potential with respect to said anode and said grid at a negative potential with respect to said cathode whereby electrons are discharged from said cathode, means directing a gas to be ionized into said anode whereby said gas is ionized to produce an arc discharge, means for producing a magnetic field having lines of force extending axially through said anode and radially diverging between said anode and grid whereby said are discharge is contrained to pass through said anode and to expand exterior thereto and to present an expanded cross section to said grid for ion ejection by said grid.

15. An improved ion source comprising means establishing an arc discharge, means establishing a magnetic field having lines of force collimating said are discharge over a portion of the length thereof and diverging to expand the arc discharge at one end thereof, an apertured grid element disposed. at the expanded end of said arc, and means maintaining said grid at a negative potential to attract ions from the expanded end of said are.

16. An improved ion source as defined in claim 15 further characterized by means substantially enclosing said collimated arc discharge and having a grid disposed across said collimated arc to limit gas flow from said source and means evacuating the region of expanded arc whereby the ratio of ionized to unionized gas leaving said source is maximized.

17. An improved ion source comprising means establishing a magnetic field, a cylindrical anode axially aligned with said magnetic field, a cathode disposed adjacent one end of said anode, a grid disposed at a distance from the other end of said anode in axial alignment therewith, means supplying a gas to be ionized to said anode, power supply means maintaining said cathode negative with respect to said anode and said grid negative with respect to said cathode whereby electrons are emitted from said cathode to produce an arc discharge through said anode, means terminating the extent of said magnetic field between said anode and grid whereby said are expands to present a large cross section to said grid for ion removal therefrom, an exit grid disposed across the end of said anode closest to said grid to limit gas flow therethrough and to define the arc distribution exterior to said anode, and means exhausting the space between said anode and grid whereby the area of ion removal in relation to the area of gas escape is maximized.

18. An improved ion source comprising means including a cathode, an apertured anode, and a magnetic field acting axially thereof for producing a collimated are discharge through said anode, an exit grid disposed across said are discharge and having a radially increasing transparency, means establishing a magnetically field free region on the opposite side of said exit grid from said anode whereby said are expands to an increased cross section in said region, and a grid structure disposed in said region in alignment with said anode and being mainpotential with respect to said are whereby the amount of 10 ions attracted from said are thereby is substantially proportional to the area thereof exposed to said grid structure and the ions so attracted are focused by said second grid.

References Cited in the file of this patent UNITED STATES PATENTS Smith May 16, 1950 Hernqvist Oct. 2, 1951 

